By: Dave Renetzky, HNTB Corporation
Intensive Vegetative Roof Example |
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Source: Roofscapes, Incorporated (2005) |
One sustainable stormwater management technique which continues to grow in popularity among commercial development is the application of green roof technologies. This involves replacing or supplementing a traditional roofing material with vegetation. The vegetation prevents the rain from quickly running down the roof surface and offsite. Water instead percolates through soil and feeds plants in the process. The vegetative roof system provides a storage component and enhances the quality of runoff through natural filtration. In general, the system is composed of a waterproofing layer, a soil layer, and a plant layer. A vegetated roof is obviously easiest to create on a minimal slope, but it can be created on surfaces with a pitch of up to 45 degrees.
Three distinct types of green roofs allow conservation of pervious area otherwise replaced by a building footprint: intensive, extensive, and modular block. Intensive vegetative roof systems require at least one foot of soil depth to create a roof garden
Extensive Vegetative Roof Example |
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Source: Roofscapes, Incorporated (2005) |
with large trees, shrubs, and other landscapes. They require irrigation and drainage systems to maintain the garden, which can add considerable load to the roof. Intensive green roofs add about 80 to 150 pounds per square foot to the building, but are usually accessible as a park or building amenity. The photo at top right provides a good example of an intensive green roof system.
In contrast to the intensive design, extensive roof systems are primarily built for the environmental benefit. They typically use only 1 to 5 inches of soil depth and contain small, hardy plants and thick grasses which are accessed for maintenance only. Additional loads to a building are between 15 and 50 pounds per square foot for the extensive system. The photo at middle right shows a typical extensive vegetative roof system.
The third type of green roof is a modular block system having self-contained portable
Modular Block Vegetative Roof Example |
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Source: St. Louis Metalworks Company (2005) |
units which are simply placed on the roof. The blocks are usually a heavy gauge aluminum container with about 4 inches of soil supporting a low-growing plant species. A pad is fastened to the bottom of the block to protect the roof surface and allow controlled drainage through the unit. The typical weight of the system is 12 to 18 pounds per square foot. The photo at bottom right demonstrates the placement of a modular unit on a high-pitched roof.
The cost of a green roof system can vary considerably depending upon the type of installation and factors such as the overall size, plant selection, depth of growing medium, use of irrigation, and accessibility options. According to the Green Roof Infrastructure Industry, an intensive or extensive system with a waterproof membrane may be installed for $9-$24 per square foot. The modular block system has an average installation cost of about $13 per square foot (St. Louis Metalworks Company). It is important to note that, while green roof technology typically requires a greater initial investment, the system can significantly extend the life of the roof and reduce the heating and cooling costs of a building. As with many sustainable management practices, one must look at a life-cycle analysis to truly compare costs. The stormwater quantity and quality benefits are more obvious.